Mist generating apparatus and method

ABSTRACT

An apparatus for generating a mist is provided. The apparatus has at least one working fluid supply conduit having an inlet in fluid communication with a supply of working fluid and an outlet in fluid communication with a first mixing chamber. The apparatus also includes a plurality of transport fluid passages, each of which has an inlet adapted to receive a supply of transport fluid and an outlet in fluid communication with the mixing chamber. Downstream of the mixing chamber is a nozzle having an inlet in fluid communication with the mixing chamber, an outlet, and a throat portion intermediate the nozzle inlet and outlet. The throat portion of the nozzle has a cross sectional area which is less than that of either the nozzle inlet or the nozzle outlet. The apparatus enhances the atomization of the working fluid to generate the mist.

The present invention provides an improved apparatus and method forgenerating mists of very small droplets, which have been shown to bebeneficial in a number of diverse fields. Examples of such fieldsinclude cooling, fire suppression and decontamination applications.

W0 01/76764 discloses a mist generating apparatus which uses two fluids,primarily for use in fire suppression. In WO '764 an aerosol of firstfluid droplets (i.e. droplets of a first fluid carried in a gaseousmedium) is passed through a number of first fluid nozzles into a mixingzone. At the same time, a stream of gas is injected into the mixing zoneupstream of the first fluid nozzles. The gas carries the first fluiddroplets through an outlet nozzle which sprays the combined stream offirst fluid droplets and second fluid from the apparatus. The purpose ofWO '764 is to reduce the frictional forces which act on the dropletswhen they are sprayed into the atmosphere by carrying the droplets outof the nozzle on the gas stream.

WO '764 only uses the gas stream to carry the droplets out of thenozzle. The aerosol of first fluid droplets is created at an undisclosedlocation upstream of the WO '764 apparatus, and the apparatus itselfdoes not apply any mechanism to further atomise the droplets of thefirst fluid in the aerosol. Consequently, the aerosol created upstreamof the WO '764 apparatus dictates the size of the droplets sprayed fromthe apparatus, with the apparatus itself having no effect on the dropletsize. A further limitation of the WO '764 apparatus is that it isdifficult to achieve a homogenous mixture of droplets and gas. The firstembodiment disclosed in WO '764 relies on a single, annular stream ofgas which is positioned radially outward of the first fluid passage andnozzles. This arrangement makes it highly unlikely that an effectivedistribution of first fluid droplets in the gas will be achieved. Suchlimitations make unpredictable variations in droplet size anddistribution very likely with the arrangement shown in WO '764.

It is an aim of the present invention to obviate or mitigate these andother disadvantages with the prior art.

According to a first aspect of the present invention, there is providedan apparatus for generating a mist, comprising:

at least one working fluid supply conduit having an inlet in fluidcommunication with a supply of working fluid and an outlet;

a first mixing chamber being in fluid communication with the workingfluid supply conduit outlet;

a plurality of transport fluid passages, each transport fluid passagehaving an inlet adapted to receive a supply of transport fluid and anoutlet in fluid communication with the mixing chamber; and

-   -   a nozzle having an inlet in fluid communication with the mixing        chamber, an outlet, and a throat portion intermediate the nozzle        inlet and outlet, the throat portion having a cross sectional        area which is less than that of either the nozzle inlet or the        nozzle outlet.

The apparatus may further comprise at least one working fluid passageintermediate the working fluid supply conduit and the mixing chamber,wherein the working fluid passage has an inlet in fluid communicationwith the supply conduit and a diameter which is less than that of thesupply conduit.

The apparatus has a longitudinal axis, and at least one of the transportfluid passage outlets may be positioned a shorter radial distance fromthe longitudinal axis than the working fluid passage outlet.

The plurality of transport fluid passages may comprise an innertransport fluid passage co-axial with the longitudinal axis, and aplurality of outer transport fluid passages circumferentially spacedabout the inner transport fluid passage.

The apparatus may comprise a plurality of working fluid passages,wherein the working fluid and transport fluid passages alternatecircumferentially about the longitudinal axis of the apparatus.

The apparatus may comprise a plurality of working fluid passages,wherein the working fluid passages are circumferentially spaced aboutthe inner transport fluid passage. The working fluid passages may beradially positioned between the inner transport fluid passage and theouter transport fluid passages. Alternatively, each of the working fluidpassages may be located between a pair of the outer transport fluidpassages, whereby the working fluid and outer transport fluid passagesalternate circumferentially about the inner transport fluid passage.

The plurality of working fluid passages may comprise inner and outerworking fluid passages, wherein the groups of inner and outer workingfluid passages are both circumferentially spaced about the innertransport fluid passage, the outer working fluid passages being agreater radial distance from the inner transport fluid passage than theinner working fluid passages.

The working fluid and transport fluid passages may be substantiallyparallel to one another.

The at least one working fluid passage is substantially parallel to thelongitudinal axis of the apparatus.

The working fluid supply conduit and the working fluid passage may besubstantially perpendicular to one another.

The apparatus may further comprise a second mixing chamber intermediatethe working fluid supply conduit and the first mixing chamber, whereinat least one of the transport fluid passages is in fluid communicationwith the second mixing chamber whilst the remainder of the transportfluid passages are in fluid communication with the first mixing chamber.

The apparatus may further comprise a communicating passageway betweenthe first and second mixing chambers, the passageway having a crosssectional area which is less than that of either mixing chamber.

According to a second aspect of the present invention, there is providedan apparatus for generating a mist comprising:

a body having a first end in which a working fluid inlet and a transportfluid inlet are defined and a second end in which a compartment isdefined, the compartment having a first end in fluid communication withthe working and transport fluid inlets and a second end which is open;

a first insert adapted to be received within the open end of thecompartment, the first insert defining at least one working fluid supplyconduit in fluid communication with the working fluid inlet, and aplurality of transport fluid passages in fluid communication with thetransport fluid inlet;

a second insert adapted to be received in the compartment between thefirst insert and the open end of the compartment, wherein the secondinsert defines a nozzle having a throat portion of reduced crosssectional area, and wherein the first and second inserts define a firstmixing chamber between them which is intermediate the working andtransport fluid passages and the nozzle; and

a locking member adapted to be received on the second insert and thesecond end of the body so as to secure the first and second inserts inthe compartment.

The first insert may further comprise at least one working fluid passageintermediate the working fluid supply conduit and the first mixingchamber, the working fluid passage having an inlet in fluidcommunication with the supply conduit and having a diameter which isless than that of the supply conduit.

The apparatus and first insert are co-axial about a longitudinal axis,and at least one of the transport fluid passage outlets defined in thefirst insert may be positioned a shorter radial distance from thelongitudinal axis than the working fluid passage outlet.

The plurality of transport fluid passages defined in the first insertmay comprise an inner transport fluid passage co-axial with thelongitudinal axis, and a plurality of outer transport fluid passagescircumferentially spaced about the inner transport fluid passage.

The first insert may define a plurality of working fluid passages,wherein the working fluid and transport fluid passages alternatecircumferentially about the longitudinal axis of the first insert.

The first insert may define a plurality of working fluid passages,wherein the working fluid passages are circumferentially spaced aboutthe inner transport fluid passage. The working fluid passages may beradially positioned between the inner transport fluid passage and theouter transport fluid passages. Alternatively, each of the working fluidpassages may be located between a pair of the outer transport fluidpassages, whereby the working fluid and outer transport fluid passagesalternate circumferentially about the inner transport fluid passage.

The plurality of working fluid passages may comprise inner and outerworking fluid passages, wherein the groups of inner and outer workingfluid passages are both circumferentially spaced about the innertransport fluid passage, the outer working fluid passages being agreater radial distance from the inner transport fluid passage than theinner working fluid passages.

The working fluid and transport fluid passages defined by the firstinsert may be substantially parallel to one another.

The at least one working fluid passage is substantially parallel to thelongitudinal axis of the first insert.

The working fluid supply conduit and the working fluid passage may besubstantially perpendicular to one another.

The first insert may further comprise a second mixing chamberintermediate the working fluid supply conduit and the first mixingchamber, wherein at least one of the transport fluid passages is influid communication with the second mixing chamber whilst the remainderof the transport fluid passages are in fluid communication with thefirst mixing chamber.

The apparatus may further comprise a communicating passageway betweenthe first and second mixing chambers, the passageway having a crosssectional area which is less than that of either mixing chamber.

According to a third aspect of the present invention, there is provideda method of generating a mist, comprising the steps of:

supplying a pressurised working fluid to at least one working fluidsupply conduit; introducing a supply of transport fluid through aplurality of transport fluid passages into a first mixing chamberdownstream of the working fluid supply conduit;

atomising the working fluid by injecting a stream of working fluid fromthe working fluid supply conduit into the first mixing chamber to form adispersed phase of working fluid droplets;

directing the transport fluid and dispersed phase of working fluid fromthe first mixing chamber through a nozzle throat portion having areduced cross sectional area; and spraying the transport fluid anddispersed phase of working fluid from a nozzle outlet having a greatercross sectional area than the nozzle throat.

The mixing chamber has a longitudinal axis, and a portion of thetransport fluid may be introduced into the mixing chamber at a positionwhich is a smaller radial distance from the longitudinal axis than thatat which the working fluid is introduced.

A portion of the transport fluid may be introduced into the mixingchamber via an inner transport fluid passage which is co-axial with thelongitudinal axis, and the remainder of the transport fluid may beintroduced via a plurality of outer transport fluid passagescircumferentially spaced about the inner transport fluid passage.

The working fluid may be atomised by passing the working fluid through aplurality of working fluid passages which alternate circumferentiallywith the plurality of transport fluid passages about the longitudinalaxis.

The working fluid may be atomised by passing the working fluid through aplurality of working fluid passages which are circumferentially spacedabout the inner transport fluid passage. The working fluid passages maybe radially positioned between the inner transport fluid passage and theouter transport fluid passages. Alternatively, each working fluidpassage may be positioned between a pair of outer transport fluidpassages, whereby the working fluid and outer transport fluid passagesalternate circumferentially about the inner transport fluid passage.

According to a fourth aspect of the present invention, there is providedan apparatus for generating a mist, comprising:

at least one working fluid supply conduit having an inlet in fluidcommunication with a supply of working fluid and an outlet;

at least one transport fluid supply conduit having an inlet in fluidcommunication with a supply of transport fluid and an outlet;

a first mixing chamber being in fluid communication with the respectiveoutlets of the working and transport fluid supply conduits;

a second mixing chamber being in fluid communication with the firstmixing chamber;

a plurality of communicating passages connecting the first and secondmixing chambers; and

a nozzle having an inlet in fluid communication with the second mixingchamber, an outlet, and a throat portion intermediate the nozzle inletand outlet, the throat portion having a cross sectional area which isless than that of either the nozzle inlet or the nozzle outlet.

The apparatus may further comprise at least one working fluid passageintermediate the working fluid supply conduit and the first mixingchamber, wherein the working fluid passage has an inlet in fluidcommunication with the supply conduit and a diameter which is less thanthat of the supply conduit.

The at least one working fluid passage and transport fluid supplyconduit communicate with the first mixing chamber from substantiallyopposite directions.

The plurality of communicating passages may comprise an innercommunicating passage co-axial with the longitudinal axis, and aplurality of outer communicating passages circumferentially spaced aboutthe inner communicating passage.

A preferred embodiment of the present invention will now be described,by way of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a longitudinal section through a body or housing of a mistgenerating apparatus;

FIGS. 2A-2C are first end, longitudinal section, and second end views ofa first insert of a mist generating apparatus;

FIG. 3 is a longitudinal section through a second insert of a mistgenerating apparatus;

FIG. 4 is a longitudinal section through a locking member of a mistgenerating apparatus;

FIG. 5 is a longitudinal section through a first embodiment of a mistgenerating apparatus incorporating the components shown in FIGS. 1-4.

FIG. 6 is a longitudinal section through a second embodiment of a mistgenerating apparatus;

FIG. 7 is a longitudinal section through a third embodiment of a mistgenerating apparatus;

FIG. 8 is a longitudinal section through a fourth embodiment of a mistgenerating apparatus;

FIG. 9 is a longitudinal section through a modified first insert of amist generating apparatus; and

FIG. 10 is a schematic illustrating an equivalent angle of expansion forthe nozzle used in the various embodiments of the mist generatingapparatus.

A mist generating apparatus is generally designated 10 and is made up offour main components, which are illustrated in FIGS. 1-4.

The first component as shown in FIG. 1 is a generally cylindrical bodyor housing 20 having first and second ends 22, 24. A neck portion 26projects longitudinally from the first end 22 of the body 20. At thesecond end 24 of the body is a compartment 28 which is open at thesecond end 24 of the body 20 and adapted to receive other components ofthe apparatus 10, as will be described below. Extending longitudinallythrough the body 20 is a first supply conduit, or transport fluid supplyconduit, 30. The transport fluid supply conduit 30 has an inlet 32 inthe neck portion 26, and an outlet 34 which opens into the compartment28. The transport fluid supply conduit 30 has a diverging profile, wherethe cross sectional area of the conduit 30 increases as it extendsthrough the body 20 from the inlet 32 towards the outlet 34. A secondsupply conduit, or working fluid supply conduit, 36 is also provided inthe body 20 and extends through a side wall of the body 20. The workingfluid supply conduit 36 has an inlet 38 on the exterior of the body 20and an outlet 40 which opens into the compartment 28. Thus, thetransport and working fluid supply conduits 30, 36 are substantiallyperpendicular to one another. The neck portion 26 and/or the inlet 32are adapted so they can be connected to a source of transport fluid (notshown), while the working fluid inlet 38 is adapted so that it may beconnected to a source of working fluid (not shown). The second end 24 ofthe body 20 has a projecting lip portion 42 of reduced outside diameter,where at least a part of the outer surface of the lip portion 42 isprovided with a thread (not shown).

Two other components forming part of the apparatus are a first, or fluiddistribution, insert 50 and a second, or nozzle, insert 70, which areshown in FIGS. 2A-2C and 3 respectively and are adapted to be locatedwithin the compartment 28 of the body 20. Referring to FIGS. 2A-2C, thefirst insert 50 is a generally cylindrical insert which is 1-shaped whenviewed in a vertical section, as clearly seen in FIG. 2B. In otherwords, the first insert 50 is thickest at its outer periphery with thecentral portion of the insert 50 having a reduced thickness bycomparison. The insert 50 has a first end face 52 and a second end face54, each of which can be seen in the respective views of FIGS. 2A and2C. Each of the end faces 52, 54 of the insert 50 has an annular groove56, 57 extending about the circumference of the outer periphery of theinsert 50. Located in each of the annular grooves 56,57 is an O-ringseal 58, 59.

Because the insert 50 has an I-shape when viewed in a vertical section,the first and second end faces 52,54 of the insert 50 have first andsecond concave cavities 53,55, respectively, formed therein. Extendinglongitudinally through the insert 50 and fluidly connecting the firstand second cavities 53,55 are a plurality of first passages, ortransport fluid passages, 60 a,60 b. An inner first passage 60 a islocated in the centre of the insert 50 such that it is co-axial with alongitudinal axis L shared by the insert 50 and the assembled apparatus10. The outer first passages 60 b are circumferentially spaced about,and substantially parallel with, the inner first passage 60 a and thelongitudinal axis L.

The insert 50 also has an outer circumferential surface 62 in which achannel 64 is formed. The channel 64 extends around the entirecircumference of the insert 50. Extending radially inwards through theinsert 50 from the channel 64 are a plurality of working fluid supplyconduits 66. The supply conduits 66 are substantially perpendicular tothe first passages 60 and longitudinal axis L. The supply conduits 66extend radially inwards through the insert 50 in the circumferentialspaces provided between the outer first passages 60 b. The supplyconduits 66 allow fluid communication between the channel 64 and aplurality of second passages, or working fluid passages, 68 a,68 blocated at the radially innermost end of the conduits 66. The secondpassages are divided into two groups whereby there are a plurality ofinner second passages 68 a and a plurality of outer second passages 68b. Each of the second passages 68 a,68 b is substantially parallel withthe longitudinal axis L and the first fluid passages 60 a, 60 b and thussubstantially perpendicular to the supply conduits 66. The secondpassages 68 a,68 b have a substantially constant diameter which may beless than that of the supply conduits 66. The inner and outer secondpassages 68 a,68 b are circumferentially spaced about the inner firstpassage 60 a and axis L, with the outer second passages 68 b beinglocated radially outwards of the inner second passages 68 a. The secondpassages 68 a,68 b are substantially parallel to the longitudinal axisL, as well as the first passages 60 a, 60 b.

The relative radial and circumferential positions of each of the firstand second passages can be best seen in FIG. 2C. From FIG. 2C, it can beseen that the second passages 68 a, 68 b are radially andcircumferentially spaced so as to surround the inner first passage 60 a,whilst the outer first passages 60 b are radially and circumferentiallyspaced so as to surround the second passages 68 a, 68 b.

The second nozzle insert 70 can be seen in FIG. 3. As with the firstinsert 50, the second insert 70 is generally cylindrical and is co-axialwith the remaining components of the apparatus 10. The second insert 70has a nozzle 72 defined therein, the nozzle 72 having a nozzle inlet 74,a throat portion 76 and a nozzle outlet 78. The nozzle 72 is co-axialwith the axis L, and the throat portion 76 intermediate the nozzle inlet74 and nozzle outlet 78 has a cross sectional area which is less thanthat of either the nozzle inlet 74 or the nozzle outlet 78. It can alsobe seen clearly from FIG. 3 that the reduction and subsequent increasein cross sectional area through the nozzle 72 maintains a continuouslyvarying external wall in the nozzle 72. In other words, the nozzle 72does not include any sudden step changes in cross sectional area, whichwould create steps or niches in the nozzle wall which would interferewith the fluid flow therethrough. The nozzle 72 is therefore a genuineconvergent-divergent nozzle as is understood in the art as beingsuitable for generating supersonic flow therethrough.

The nozzle insert 70 has first and second ends having a first end face71 and a second end face 73, respectively. A groove 80 is located in theouter circumferential surface of the insert 70 adjacent the first end.The groove 80 extends around the entire circumference of the insert 70and an o-ring seal 82 is located in the groove 80. The nozzle insert 70has a reduced diameter portion 75 adjacent the second end. The variationbetween the standard diameter of the insert 70 and the reduced diameterportion 75 creates an abutment face 77, which faces in the direction ofthe second end of the insert 70.

The final component of the apparatus 10 is a locking member 90, which isshown in FIG. 4. The locking member 90 is preferably in the form of aring which has a first side face 92 and a second side face 94. Thelocking member 90 has a bore passing through it which is formed fromfirst and second portions 96,98. The first bore portion 96 opens on thefirst side face 92 whilst the second bore portion 98 opens on the secondside face 94. The first bore portion 96 has a greater diameter than thesecond bore portion 98. The variation in diameter between the first andsecond bore portions 96,98 creates an abutment face 100, which faces inthe direction of the first side face 92 of the locking member 90. Atleast a part of the internal surface of the first bore portion 96 isprovided with a thread (not shown). The second end 94 of the lockingmember 90 can be provided with one or more apertures 102 adapted toreceive a suitable tool for securing the locking member 90 to theremainder of the apparatus 10.

Referring now to FIG. 5, the various components of the apparatus 10 asdescribed above are assembled in the following manner. Firstly, thefluid distribution insert 50 is slid into the compartment 28 via thesecond end 24 of the body 20. The internal diameter of the compartment28 and the external diameter of the insert 50 are such that a close,sealing fit is achieved between the insert 50 and the body 20. When theinsert 50 is correctly positioned within the compartment 28, the firstend face 52 of the insert abuts the outlet 34 of the transport fluidsupply conduit 30 in the body 20. As a result, the outlet 34 of thetransport fluid supply conduit 30 is in fluid communication with thefirst cavity 53 of the insert 50, and the second fluid supply conduit 36is in fluid communication with the channel 64 of the insert 50. Theo-ring seal 58 provides a sealing fit between the first insert 50 andthe body 20.

Once the first insert is in position, the second insert 70 can beinserted into the compartment 28 via the second end 24 of the body 20.As with the first insert 50, the internal diameter of the compartment 28and the external diameter of the second insert 70 are such that a close,sealing fit is achieved between the insert 70 and the body 20. When thesecond insert 70 is correctly positioned within the compartment 28, thefirst end face 71 of the second insert 70 abuts the second end face 54of the first insert 50. As a result, a mixing chamber sharing thelongitudinal axis L is defined by the nozzle inlet 74 of the secondinsert 70 and the second cavity 55 of the first insert 50. Consequently,the body 20, first insert 50 and second insert 70 are now all in fluidcommunication with one another via the previously described cavities,passages and conduits defined within these components, as will bedescribed in further detail below. The second of the O-ring seals 59located in the second end face 54 of the first insert 50 provides asealing fit between the first and second inserts 50, 70.

Finally, once the first and second inserts 50,70 are located in theircorrect positions in the compartment 28 of the body 20, the lockingmember 90 can be placed over the second end of the second insert 70. Thethreaded portions of the lip 42 of the body 20 and the first side face92 of the locking member 90 cooperate with one another so that thelocking member 90 can be screwed into position by way of a tool (notshown) inserted into the apertures 102 in the locking member 90. Thelocking member 90 is screwed onto the body 20 until the respectiveabutment faces 77, 100 of the second insert 70 and the locking member 90come up against one another. Once this has taken place, the first andsecond inserts 50,70 are firmly held in position, sandwiched between thebody 20 and the locking member 90.

The manner in which the apparatus 10 operates can now be described,again with particular reference to FIG. 5. Initially, a transport fluidis introduced from a suitable source (e.g. a bottle of compressed gas)into the transport fluid supply inlet 32. There are a variety of fluidswhich would be suitable for use as the transport fluid, but in thispreferred example the transport fluid is air. The supply pressure of thetransport fluid may be in the range 2 to 40 bar, or more preferably inthe range 5 to 20 bar. The transport fluid passes along the transportfluid supply conduit 30 in the direction of the arrow T into the firstcavity 53 defined in the first insert 50. Once in the first cavity 53,the transport fluid separates into a number of flow paths as it entersthe inner and outer first fluid passages 60 a, 60 b provided in thefirst insert 50. As the transport fluid flows leave the first fluidpassages 60 a, 60 b they enter the mixing chamber defined between thesecond cavity 55 of the first insert 50 and the nozzle inlet 74 of thesecond insert 70. The various transport fluid flows expand and come intocontact with one another in the mixing chamber, thereby creating aturbulent zone in the mixing chamber. The transport fluid enters themixing chamber under high pressure but with a relatively low velocity.

At the same time as the transport fluid is being introduced into thetransport fluid supply conduit 30, a working fluid is being introducedfrom a suitable source at a preferred supply pressure in the range 2 to40 bar, most preferably in the range 5 to 20 bar. The working fluid isintroduced into the working fluid supply conduit 36 provided in the body20. As with the transport fluid, the working fluid can be a number offluids but in this preferred example is water. As the working fluidpasses through the working fluid supply conduit 36, it enters thechannel 64 provided in the exterior of the first insert 50. The workingfluid can then flow around the entire circumference of the first insert50 via the channel 64, which lies between the body 20 and the firstinsert 50. As it flows around the channel 64, the working fluid entersthe plurality of radial supply conduits 66 in the first insert 50 andflows inwards towards the longitudinal axis L of the apparatus. At theinner ends of the supply conduits 66, the working fluid turns through 90degrees and enters the inner and outer second fluid passages 68 a,68 b.This 90 degree turn destabilises the working fluid, increasing the levelof turbulence therein and enhancing the atomisation of the working fluidin the mixing chamber, which will be further described below.

The transport and working fluids can be supplied over a large range ofmass flow rates. The ratio between the mass flow rates of transport andworking fluid may vary over a preferred range from 20:1 to 1:10.

Once the working fluid reaches the outlets of the second fluid passages68 a, 68 b, a stream of working fluid is injected from each secondpassage 68 a, 68 b into the mixing chamber. As the injected workingfluid streams come into contact with the ambient gas in the mixingchamber, frictional forces between the two lead to the atomisation ofthe working fluid streams, thereby forming droplets of working fluid.The turbulence generated by the transport fluid entering the mixingchamber ensures that the droplets created by this atomisation of theworking fluid are spread throughout the mixing chamber. This is thefirst stage of the atomisation mechanism employed by the presentinvention.

The remaining stages of the atomisation mechanism occur in the nozzle 72of the apparatus 10. The working fluid droplets in the mixing chamberare carried by the turbulent transport fluid into the nozzle inlet 74.The gradual reduction in cross sectional area between the nozzle inlet74 and the nozzle throat 76 leads to an acceleration of the transportfluid to a very high, preferably sonic, velocity. This acceleration ofthe transport fluid means that there is a velocity gradient across thedroplets of working fluid in the convergent region of the nozzle (ie.the region between the nozzle inlet and the nozzle throat), as theportion of each droplet closest to the nozzle throat will be travellingfaster than the portion closest to the nozzle inlet. This subjects theworking fluid droplets to shear forces and leads to them stretching orelongating in the direction of flow. When the shear forces exceed thesurface tension forces a further atomisation occurs as the dropletsdeform and break up into smaller droplets. This shearing action is thesecond stage of the atomisation mechanism.

The reduced size working fluid droplets leave the nozzle throat 76 atvery high, and possibly sonic, velocity. As previously described, thenozzle outlet 78 has a greater cross sectional area than the nozzlethroat 76. Consequently, the high velocity transport fluid undergoes anexpansion as it flows from the throat portion 76 towards the outlet 78.This stretches the working fluid droplets contained in the transportfluid and causes them to break up into a number of smaller working fluiddroplets. This tearing of the droplets is the third stage in theatomisation mechanism employed by the present invention.

Finally, the droplets are sprayed from the nozzle outlet 78 in adispersed phase as a mist. Depending on the operating conditions, theflow through the nozzle 72 may be subsonic in the region between thethroat portion 76 and the nozzle outlet 78. Alternatively, the operatingconditions may mean that the flow in this region may be supersonic alongsome or all of its length, with the supersonic region terminating in ashock wave either between the throat portion 76 and the nozzle outlet78, at the nozzle outlet 78, or external to the apparatus 10. In thoseoperating conditions at which a shock wave occurs, it may provide afourth droplet breakup mechanism due to the sudden pressure rise acrossthe shockwave.

FIG. 10 shows schematically how an equivalent angle of expansion for thenozzle 72 can be calculated when the cross sectional areas of the throatand outlet, and the equivalent path distance between the throat andoutlet are known. E1 is the radius of a circle having the same crosssectional area as the nozzle throat 76. E2 is the radius of a circlehaving the same cross sectional area as the nozzle outlet 78. Thedistanced is the equivalent path distance between the throat 76 and theoutlet 78. An angle β is calculated by drawing a line through the top ofE2 and E1 which intersects a continuation of the equivalent distanceline d. This angle β can either be measured from a scale drawing or elsecalculated from trigonometry using the radii E1, E2 and the distance d.The equivalent angle of expansion y for the second fluid passage canthen be calculated by multiplying the angle β by a factor of two, wherey=2β.

For optimum performance of the apparatus 10, it has been found that thecross sectional area at the outlet 78 of the nozzle 72 may be between1.1 and 28 times larger than that of the throat portion 76, such thatthe area ratio between the throat 76 and outlet 78 of the nozzle 72 maybe between 1:1.1 and 1:28. The cross sectional area at the outlet 78 ofthe nozzle 72 may most preferably be between 1.4 and 5.5 times largerthan that of the throat portion 76, such that the area ratio between thethroat 76 and outlet 78 of the nozzle 72 is therefore most preferablybetween 5:7 and 2:11. This increase in cross sectional area between thethroat 76 and outlet 78 creates an equivalent included angle ofexpansion y for the nozzle 72 of between 1 and 40 degrees, and an angley which is most preferably between 2 and 13 degrees.

Performance data obtained in tests of the apparatus shown in FIG. 5 ispresented in Table 1 below. The results were obtained using a laserdiffraction particle size system which measures the droplet sizes andperforms the data analysis. The data was measured 3 m from the nozzle inthe centre of the plume as this allowed good particle observation withthe measurement system, but also represented typical plumecharacteristics for the nozzle. Having determined the droplet sizespresent in the plume, the data was further analysed to calculate theD_(v)90 and D_(f)90, which are common measurement parameters used inindustry. The D_(v)90 is the value where 90 percent of the total volumeof the liquid sprayed is made up of drops with diameters smaller than orequal to this value. The D_(f)90 is the value where 90 percent of thetotal number of droplets sprayed have diameters smaller than or equal tothis value.

In this non-limiting test example the transport fluid utilised wascompressed air and the working fluid utilised was water.

TABLE 1 Mass Pressure flow rate, supply, ratio of ratio of Dv90 Dt90gas:liquid gas:liquid [μm] [μm] 1:4 1:0.875 180 4 1:8 1:0.875 220 2.5 1:14 1:0.861 255 2.5

FIGS. 6-8 show alternative embodiments of a mist generating apparatus.Each of these alternative embodiments utilises the first and secondinserts 50,70 and the locking member 90 as already described above withreference to FIGS. 2A-4. The features of these components have thereforebeen assigned the same reference numbers and will not be described againin connection with these alternative embodiments.

Where these alternative embodiments differ from the first embodimentdescribed above is that they are provided with a third insert which isto be located in the compartment 28 of the body 20 along with the firstand second inserts 50,70.

In the second embodiment of the apparatus 10′ shown in FIG. 6, a thirdinsert 110 is inserted into the compartment 28 prior to the insertion ofthe first and second inserts 50,70. The third insert 110 is tubular andhas an outer diameter which is selected so as to provide a close,sealing fit between the tubular member 110 and the inner surface of thecompartment 28. To assist with the sealing fit, a first end 112 of thethird insert 110 is provided with a first circumferential groove 114 inwhich an O-ring seal 116 is located. Thus, when the third insert 110 iscorrectly positioned in the compartment 28, the first end 112 and seal116 abut against the outlet 34 of the transport fluid supply conduit 30.A second circumferential groove 118 is provided in the outer surface ofthe third insert 110 adjacent a second end 113 of the insert 110. Afurther O-ring seal 117 is provided in the second groove 118 to aid thesealing of the outer surface of the third insert 110 to the innersurface of the compartment 28.

Certain modifications may be made to the body 20 in order to incorporatethe third insert 110. The axial length of the compartment 28 may beincreased so that all three inserts 50, 70,110 can be located therein.Alternatively, the axial length of the first and second inserts 50,70may be reduced in order that all three inserts may be accommodated.Another modification that may be required is to form the working fluidsupply conduit 36 at a different axial position on the body 20. Thiswill be necessary if the third insert 110 is located upstream of thefirst insert 50, as the first insert 50 will then be further along thecompartment 28 than in the first embodiment. As seen in FIG. 6, thesupply conduit 36 has been repositioned so that the first insert 50still receives the working fluid via the supply conduit 36 and thechannel 64.

The second embodiment of the apparatus 10′ is assembled and operates insubstantially the same manner as the first embodiment. However, thepresence of the tubular third insert 110 between the transport fluidsupply conduit 30 and the first insert 50 effectively increases theaxial length of the transport fluid supply conduit 30.

The third and fourth embodiments of the apparatus 10″,10′″ are shown inFIGS. 7 and 8. These embodiments are variations on the second embodimentin that they are also provided with supplementary inserts. The thirdembodiment shown in FIG. 7 has a third insert 120 substantiallyidentical to that used in the second embodiment. However, in the thirdembodiment the third insert 120 is positioned in the compartment 28 suchthat it is sandwiched between the first insert 50 and the second insert70. As with the second embodiment, the axial length of the compartment28 in the body 20 may be extended to accommodate all three inserts. Thethird embodiment is assembled and operates in substantially the samemanner as the first and second embodiments, but the presence of thetubular third insert 120 between the first and second inserts 50,70effectively increases the axial length of the mixing chamber downstreamof the first insert 50.

The fourth embodiment of the apparatus 10′″ shown in FIG. 8 effectivelycombines the arrangements used in the second and third embodiments ofthe apparatus. This results in third and fourth inserts 130,140 beinglocated in the compartment 28 upstream and downstream of the firstinsert 50, respectively. The third and fourth inserts 130,140 aretubular and substantially identical to the third inserts used in thesecond and third embodiments. The only difference envisaged between theinserts of this embodiment and the third inserts of the precedingembodiments is that they may be of shorter axial length so that all fourinserts fit in the compartment 28 of the body 20. Again, the body 20 maybe modified to vary the axial length of the compartment 28 and/or axiallocation of the working fluid supply conduit 36 according to thepositions of the inserts.

The fourth embodiment is assembled and operates in substantially thesame manner as the preceding embodiments, but the presence of both thirdand fourth tubular inserts 130,140 either side of the first insert 50effectively increases the axial length of both the transport fluidsupply conduit 30 and the mixing chamber downstream of the first insert50.

Using these supplementary third, or third and fourth, inserts of varyinglengths reduces the manufacturing complexity of the apparatus. Forinstance different sizes and lengths of nozzle, or first insert, couldbe installed into the apparatus body along with one or moresupplementary inserts without the need to modify the length of the bodyor the locking member, or to change the pipework connecting it to aworking fluid source. Additionally, changing the axial length of themixing chamber(s) may alter the turbulence in these regions and alterthe first stage of the atomisation mechanism employed by the presentinvention.

FIG. 9 shows a section view of a modified first insert 150, which couldbe utilised in any of the preceding embodiments of the mist generatingapparatus. The basic configuration of the modified first insert 150 issubstantially the same as the first insert 50 of FIGS. 2A-2C, with firstand second cavities 53,55 being fluidly connected with one another by aplurality of first passages, or transport fluid passages, 60 a, 60 b. Aninner first passage 60 a is located in the centre of the modified insert150 such that it is co-axial with a longitudinal axis L shared by theinsert 150 and the assembled apparatus in which it will be located. Theouter first passages 60 b are circumferentially spaced about, andsubstantially parallel with, the inner first passage 60 a and thelongitudinal axis L.

The modified insert 150 also has an outer circumferential surface 62 inwhich a channel 64 is formed. The channel 64 extends around the entirecircumference of the insert 50. Extending radially inwards through theinsert 50 from the channel 64 are a plurality of working fluid supplyconduits 66. The supply conduits 66 are substantially perpendicular tothe first passages 60 a,60 b and longitudinal axis L. The supplyconduits 66 extend radially inwards through the insert 50 in thecircumferential spaces provided between the outer first passages 60 b.Where the modified insert 150 differs from the original first insert isthat the second, or working fluid passages, have been replaced with acentral third cavity 170. The third cavity 170 is co-axial with thelongitudinal axis L and the inner first passage 60 a. The third cavity170 is formed such that it is in fluid communication with the innerfirst passage 60 a, each of the supply conduits 66 and the second cavity55. The third cavity 170 has an internal diameter which is larger thanthat of the inner first passage 60 a but smaller than that of the secondcavity 55. A circumferential lip 172 projects radially inwards from thewall of the third cavity 170 at the point where the third cavity opensinto the second cavity 55.

A substantially circular plug 152 is provided for insertion into thethird cavity 170 from the second cavity 55. The plug 152 has a plug body153 whose external diameter is greater than the internal diameter of thelip 172. Therefore when the plug 152 is inserted into the third cavity170 the plug body 153 pushes past the lip 172 and there is a snap-fitbetween the plug body 153 and the lip 172. The lip 172 thus prevents theplug 152 from coming out of the cavity 170. A flange portion 154projects radially outwards from the plug body 153. The flange portion154 has a larger diameter than the internal diameter of the third cavity170 so as to limit the extent to which the plug 152 may enter the thirdcavity 170.

A central passage extends longitudinally through the plug 152. Thecentral passage comprises a large diameter portion 160 a and a smalldiameter portion 160 b. When the plug 152 is in position within themodified insert 150, the third cavity 170 and the large diameter portion160 a of the central passage define a first stage mixing chamber 151.The first stage mixing chamber 151 will receive transport fluid from theinner first passage 60 a and working fluid from the supply conduits 66.The small diameter portion 160 b of the central passage allows thetransport and working fluids received by the first stage mixing chamber151 to pass into the main mixing chamber partially defined by the secondcavity 55.

Transport fluid passing from the relatively small diameter inner firstpassage 60 a into the larger diameter first stage mixing chamber 151will expand and create a turbulent flow within the first stage mixingchamber. Working fluid entering the first stage mixing chamber 151 willencounter this turbulence and the frictional forces generated betweenthe two fluids will lead to the atomisation of at least some of theworking fluid. The flow of transport and working fluids will then passthrough the small diameter portion 160 a of the central passage into themain mixing chamber downstream. Thus, the modified first insert 150provides an initial mixing stage for the transport fluid and workingfluid before the main mixing stage which takes place downstream of thefirst insert, as described above. This initial mixing stage enhances theatomisation mechanisms occurring upstream of the nozzle by providing atwo stage initial atomisation process of turbulent mixing and dropletbreakup.

Providing a plurality of transport fluid passages allows the formationof a number of separate transport fluid flow paths into the mixingchamber. When these various transport fluid flows contact one another inthe mixing chamber, a greater amount of turbulence is created in themixing chamber. The enhanced turbulence ensures that the atomiseddroplets are evenly distributed throughout the mixing chamber.Additionally, the high levels of turbulence mean that if dropletscollide with one another, or a surface, the generated internal stresseswill be high, such that they are more likely to exceed the surfacetension forces. This means that collisions are more likely to causedroplet breakup rather than coalescence. Arranging the various passagesso that the transport fluid outlets surround the working fluid outlets,whether in the radial or circumferential direction, achieves a morehomogenous distribution of droplets in the mixing chamber and expansionsection (i.e. post-throat portion) of the nozzle. This ensures that thethird (expansion) stage of the atomisation process is as effective aspossible.

When present, a plurality of working fluid passages allows a greaterflowrate of working fluid to be atomised.

Positioning the working fluid passage outlets towards the outside of themixing chamber can enhance atomisation by optimising a wall strippingmechanism. With wall stripping, a film of working fluid which attachesitself to the inner surface of the mixing chamber will be graduallyatomised as the transport fluid flow strips droplets from the film ofworking fluid. Providing a longer mixing chamber, as in the case of thethird embodiment using a third insert, can enhance the wall strippingprocess, as the surface area over which the film of working fluidextends is increased.

The transport fluid supply conduit, the transport fluid passages and thenozzle passage are relatively wide and have minimal restrictionstherein. As a result, a particulate-laden fluid can be used as thetransport fluid without any concerns that the relevant passages willbecome blocked by the particulate matter contained in the transportfluid.

By forming the apparatus from a small number of components, the presentinvention provides a simplified manufacturing process. The individualcomponents themselves are of a reduced complexity compared with existingapparatus, which is advantageous in terms of production costs.Additionally, as the inserts are fitted in the body and held in place bythe locking member, the machining tolerances required when manufacturingthe components can be reduced.

The outer first fluid passages need not be parallel to the longitudinalaxis L. Instead the outer first fluid passages may be angled relative tothe longitudinal axis L. In other words, the inlet and outlet of eachouter first fluid passage may be at different radial positions relativeto the axis L. Furthermore, the first fluid passages need not be ofsubstantially constant diameter. The first fluid passages may have aportion which is of reduced diameter and/or a portion which is ofincreased diameter. As well as a generally teardrop cross section, thefirst fluid passages may alternatively have a substantially circularcross section, or they may have an elliptical cross section.

There may be more than two sets of first fluid passages. For example, athird set of first fluid passages may extend circumferentially about theinner and outer first fluid passages, at a greater radial distance fromthe axis L than those inner and outer first fluid passages.

Whilst preferable, the second fluid passages need not be locatedradially between the inner and outer first fluid passages. The secondfluid passages could be located radially and circumferentially so thatthey are between pairs of the outer first fluid passages, so that thesecond fluid passages and outer first fluid passages alternate in thecircumferential direction about the longitudinal axis L. In other words,the outlets of the second fluid passages are surrounded in thecircumferential direction by the outlets of the first fluid passages.

The second fluid passages may also be fluidly connected with the outerfirst fluid passages in the first insert such that atomisation commenceswithin the second fluid passages upstream of the mixing chamber.

Each of the second fluid passages may include a turbulence-generationcomponent therein. The component may take the form of a tapered edgeinside the passage, for example.

The second fluid passages need not be parallel to the longitudinal axisL. Instead the second fluid passages may be angled relative to thelongitudinal axis L. In other words, the inlet and outlet of each secondfluid passage may be at different radial positions relative to the axisL. Furthermore, the second fluid passages need not be of substantiallyconstant diameter. The second fluid passages may have a portion which isof reduced diameter and/or a portion which is of increased diameter.

The second fluid passages may have a substantially circular crosssection, or alternatively they may have an elliptical cross section.

There may be more than two sets of second fluid passages. For example, athird set of second fluid passages may extend circumferentially aboutthe inner and outer sets of second fluid passages, at a greater radialdistance from the axis L than the inner and outer sets of second fluidpassages.

Although the preferred embodiment of the apparatus described above hasonly one working fluid inlet in the body, there may be a plurality ofworking fluid inlets circumferentially spaced about the side wall of thebody. Each of the working fluid inlets may be in fluid communicationwith the channel extending about the circumference of the first insert.

The plug utilised in the modified first insert shown in FIG. 9 may beprovided with a plurality of supplementary passages connecting the firststage mixing chamber and the second cavity. These supplementary passagesmay be circumferentially spaced around the small diameter portion of thecentral passage. The supplementary passages may be at more than oneradial position relative to the small diameter portion of the centralpassage.

In the embodiments employing third, or third and fourth, inserts anumber of working fluid supply conduits may be supplied at variouspositions along the body. These supply conduits may be capped off orconnected to the working fluid supply as necessary, depending on theaxial location along the chamber of the first insert due to the presenceof these supplementary inserts. Alternatively, the first and thirdinserts may be shaped such that the circumferential supply channel ofthe first insert extends longitudinally and continuously over the frontportion of the first insert as well as a portion of the third insert.This would mean that a single working fluid supply conduit could beprovided in the body, but that this conduit could still provide workingfluid to the first insert when it is axially spaced from the conduit bythe presence of the third insert.

A further modification to the apparatus would be to turn the firstinsert around, such that the second fluid passages face upstream towardsthe supply of the transport fluid. In this case, working fluid andtransport fluid flowing in opposite directions would come into contactwith one another in a mixing chamber defined between the body and thefirst insert. The working fluid would be atomised in the mixing chamberand then the transport fluid would carry the dispersed working fluiddownstream to the nozzle by way of the first fluid passages in the firstinsert. The third tubular insert may also be deployed between the bodyand first insert in this modified version of the apparatus, therebyincreasing the size of the mixing chamber defined between the body andfirst insert. Extending the mixing chamber in this way can enhance theturbulent mixing therein.

In its simplest form, the apparatus of the present invention comprises aplurality of transport fluid passages and at least one working fluidpassage which open into a mixing chamber and a nozzle downstream of themixing chamber. This arrangement alone can provide one or more of thebenefits listed elsewhere in this specification. Therefore, whilst thedescription of the preferred embodiment of the present invention abovedescribes various groups of passages and their preferred radial andcircumferential positions relative to one another, it should beunderstood that these combinations are not essential for the successfuloperation of the invention. Whilst the preferred embodiment of thepresent invention described above comprises a plurality of working fluidpassages, the present invention is not limited to a number of workingfluid passages. The present invention will provide one or more of theadvantages listed herein so long as it has one or more working fluidpassages. Furthermore, whilst the preferred embodiment has an innertransport fluid passage which is co-axial with the longitudinal axis L,the present invention is not limited to the inclusion of this innertransport fluid passage. The present invention will also be effectivewith transport fluid passages which are only circumferentially spacedaround the longitudinal axis L.

As already stated in the detailed description of the present invention,the transport fluid is not limited to air. Other examples of suitablefluids are nitrogen, helium and steam. Similarly, water is not the onlysuitable working fluid which can be used with the invention. Otherfluids which include additives such as decontaminants, surfactants orsuppressants are also suitable for use as the working fluid.

These and other modifications and improvements may be incorporatedwithout departing from the scope of the invention.

1. An apparatus for generating a mist, comprising: at least one workingfluid supply conduit having an inlet in fluid communication with asupply of working fluid and an outlet; a first mixing chamber being influid communication with the working fluid supply conduit outlet; aplurality of transport fluid passages, each transport fluid passagehaving an inlet adapted to receive a supply of transport fluid and anoutlet in fluid communication with the mixing chamber; and a nozzlehaving an inlet in fluid communication with the mixing chamber, anoutlet, and a throat portion intermediate the nozzle inlet and outlet,the throat portion having a cross sectional area which is less than thatof either the nozzle inlet or the nozzle outlet. 2-30. (canceled)